In a typical phased array radar, a plurality of antenna elements are arranged in a plane, and a microwave signal of a selected frequency is fed to each antenna element. Each antenna element responds by radiating at the selected frequency. When the antenna elements radiate synchronously and inphase with one another, the radiation from the several antenna elements constructively interferes along a boresight direction normal to the planar array, and destructively interferes along other directions, thereby producing a narrow beam directed along the boresight direction. By introducing prescribed phase and/or time delays in the feed paths to the respective antenna elements, the beam can be electronically steered to angles up to about 60.degree. from the boresight direction. For two-dimensional arrays, the steering can be accomplished in two perpendicular directions normal to the boresight.
In one common application, a phased array antenna is mounted in the nose of a plane, and used to scan a cone of forward directions. For certain applications, however, it is necessary to scan a range of directions wider than that which can be scanned using a single, fixed phased array antenna. In the past, wider scanning ranges have been accomplished either by providing plural phased array antennas, or by physically rotating the entire array of antenna elements. The former approach has a disadvantage of doubling the weight and cost associated with the antennas. The latter approach is impractical in many situations, due to the size of the array and to space and/or aerodynamic considerations.